Circuit for sensing on-die temperature at multiple locations

ABSTRACT

A circuit for sensing on-die temperature at multiple locations using a minimum number of pins is described. Thermal diodes coupled to pins are placed on a die to measure the temperature at various die locations. Voltage is applied to the pins to determine the temperature at each given diode location. The polarity of the voltage applied across the pins determines what diodes are selected for measurement.

FIELD OF THE INVENTION

[0001] The present invention pertains to the field of integrated circuitdesign. More particularly, the present invention relates to a circuitfor sensing temperature at multiple locations on a silicon die using aminimum number of integrated circuit terminals.

BACKGROUND OF THE INVENTION

[0002] An integrated circuit (IC) is a device consisting of a number ofconnected circuit elements, such as transistors and resistors,fabricated on a single chip of silicon crystal or other semiconductormaterial. During operation, an IC burns power causing the temperature ofthe IC to increase. An overheated IC can potentially result in reducedperformance and even failure.

[0003] ICs, however, are typically packaged in such a way that it isdifficult to directly measure the temperature at the active part of thedie using a thermocouple or other external measuring device. As aresult, the standard method for measuring the die temperature of an ICis to incorporate a thermal diode with known thermal characteristicsinto the design of the IC.

[0004] Thermal diodes are typically the base-emitter junction of asubstrate connected PNP transistor. This junction may be modeled usingthe ideal diode equation. The equation of an ideal diode is:$\begin{matrix}{i = {I_{s}\left( {^{\frac{vq}{nkT}} - 1} \right)}} & \left( {{equation}\quad 1} \right)\end{matrix}$

[0005] where i is the forward biased current through the diode, I_(s) isthe saturation current of the diode, v is the voltage drop across thediode, q is the charge of an electron, n is an ideality factor, k isBoltzmann's Constant, and T is the temperature in Kelvin. The idealityfactor n is constant for a given process technology. A range for thisnumber is typically available in its product data sheet.

[0006] To measure the die temperature using a diode, the terminals ofthe diode are coupled to IC terminals. By applying a current i to thediode, the voltage drop v across the diode is measured at the terminals.For this measurement, the IC may or may not be running. By measuring thevoltage drop at two different currents, i₁ and i₂, at constanttemperature T, the value I_(s) is cancelled out as shown in theequation: $\begin{matrix}{\frac{i_{1}}{i_{2}} = {\frac{I_{s}\left( {^{\frac{v_{1}q}{knT}} - 1} \right)}{I_{s}\left( {^{\frac{v_{2}q}{knT}} - 1} \right)} = {\frac{^{\frac{v_{1}q}{knT}} - 1}{^{\frac{v_{2}q}{knT}} - 1}.}}} & \left( {{equation}\quad 2} \right)\end{matrix}$

[0007] Equation 2 may be simplified by removing the 1's since they arenegligible. Thus, the equation becomes: $\begin{matrix}{\frac{i_{1}}{i_{2}} = {\frac{^{\frac{v_{1}q}{knT}}}{^{\frac{v_{2}q}{knT}}}.}} & \left( {{equation}\quad 3} \right)\end{matrix}$

[0008] Given that the ratio of the currents is constant, the temperatureis directly proportional to the difference in the two measured voltagedrops, v₁ and v₂:${\ln \left( \frac{i_{1}}{i_{2}} \right)} = {{\ln \left( \frac{^{\frac{v_{1}q}{knT}}}{^{\frac{v_{2}q}{knT}}} \right)} = {{{\ln \left( ^{\frac{v_{1}q}{knT}} \right)} - {\ln \left( ^{\frac{v_{2}q}{knT}} \right)}} = {\frac{v_{1}q}{knT} - \frac{v_{2}q}{knT}}}}$

[0009] Solving for T: $\begin{matrix}{T = {\left( \frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right){\left( {v_{1} - v_{2}} \right).}}} & \left( {{equation}\quad 4} \right)\end{matrix}$

[0010] Equation 4, however, does not include the effective seriesresistance, R_(s), of the diode. Typically, long traces are a primarysource of effective series resistance. Placing the diode near ICterminals would help to reduce series resistance. In reality, however,the diode is often a distance from the IC terminals due to areaconstraints. Because the effective series resistance may be substantialin an IC such as a microprocessor, it would be desirable to include itseffects in the temperature calculation.

[0011] In addition, as the trend in IC design continues toward smallerchips, the power density increases and becomes less uniform. This causesthe thermal gradients across the die to become greater. As a result,even though previous ICs were able to suffice with a single thermaldiode at a single location on the die, future ICs may require multiplethermal diodes in order to map out the thermal profile in better detail.

[0012] Because IC terminals are at a premium on chips, the addition ofthermal diodes beyond the first one may be cost prohibitive. Therefore,it would be desirable for a circuit to allow for the placement ofmultiple thermal diodes on the die while minimizing the number of ICterminals used.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The embodiments of the present invention are illustrated by wayof example and not in the figures of the accompanying drawings, in whichlike references indicate similar elements and in which:

[0014]FIG. 1 is one embodiment of a circuit for sensing on dietemperature at multiple locations;

[0015]FIG. 2 is one embodiment of a system that measures dietemperatures; and

[0016]FIG. 3 is a flowchart for calculating the temperature at a dielocation.

DETAILED DESCRIPTION

[0017] In the following detailed description, numerous specific detailsare set forth in order to provide a thorough understanding of theinvention. However, it will be understood by those skilled in the artthat the present invention may be practiced without these specificdetails. In other instances, well-known methods, procedures, componentsand circuits have not been described in detail so as not to obscure thepresent invention.

[0018] The use of multiple thermal diodes allow for the determination ofthermal gradients on an IC. Multiple diodes may be used to measure dietemperature without having to use two times the number of IC terminalsper diode by taking advantage of the diodes' electrical properties. Adiode has an anode terminal and a cathode terminal. Current flowsprimarily from the anode terminal of the diode to the cathode terminal.

[0019] Therefore, if two diodes having opposite polarity (terminals areflipped with respect to one another) are placed between two parallelconducting wires of differing voltages, current will only flow throughthe diode having its anode terminal connected to the wire having thehigher voltage and its cathode terminal connected to the wire having thelower voltage. The other diode will not conduct because it will bereverse biased. Because the leakage current of the reverse biased diodeis substantially smaller than the current passing through the forwardbiased diode, the leakage current is negligible.

[0020] Taking advantage of this theory, FIG. 1 depicts one embodiment ofthe invention. The circuit of FIG. 1 has six diodes (101-106) and threeIC terminals (121-123). Depending on the packaging used, the ICterminals 121-123 may be implemented as bumps or pins. Diodes 101 and102 are coupled to IC terminals 121 and 122. Diodes 103 and 104 arecoupled to IC terminals 122 and 123. Diodes 105 and 106 are coupled toIC terminals 121 and 123.

[0021] Each of the diodes 101-106 has an anode terminal, a cathodeterminal, and a clamping voltage of approximately 0.7 volts (V). Whentwo voltages are applied to a diode such that the cathode terminal iscoupled to a higher voltage than the anode terminal, a forward biasedcurrent is generated across that diode.

[0022] As an example, if 0.7 V is applied to IC terminal 121, 0 V isapplied to IC terminal 123, and IC terminal 122 is left in a highimpedance state such that no current flows into or out of IC terminal122, a forward biased current will flow through diode 106. Because diode105 has the opposite polarity as diode 106, diode 105 is reverse biased.Thus, no current will flow across diode 105.

[0023] Note that diodes 102 and 104 will also be forward biased.However, because only 0.7 volts is applied across IC terminals 121 and123, the forward biased current traveling through diodes 102 and 104 areorders of magnitude less than the current traveling through diode 106.As a result, the current measured across the IC terminals 121 and 123will predominately reflect the forward biased current traveling throughdiode 106. From the current measurement, the temperature at diode 106may be calculated. The temperature calculation process is depicted inFIG. 3 and described below.

[0024] From FIG. 1 and the example above, it can be seen that a pair ofdiodes having opposite polarity may be placed between each set of ICterminals. Specifically, the number of diodes that may be used on an ICmay be defined by the equation:

D=N*(N−1)   (equation 5)

[0025] where D is the number of diodes and N is the number of ICterminals.

[0026]FIG. 2 depicts one embodiment of a system that uses thermal diodesto prevent an IC from overheating. For this embodiment of the invention,IC 100 has a plurality of diodes distributed throughout the chip tomeasure temperatures. Signals 210 and 220 are diode measurementsobtained from IC 100. A temperature sensing circuit 230 is coupled tointegrated circuit 100 and calculates temperatures based upon signals210 and 220. If the calculated temperature is greater than apredetermined threshold value, the temperature sensing circuit 230reduces the power of integrated circuit 100.

[0027] For another embodiment of the invention, the temperature sensingcircuit 230 is manufactured on the same die as integrated circuit 100.

[0028] A process for calculating IC temperatures is depicted in FIG. 3.In operation 310, a plurality of diodes is placed on an IC to determinetemperatures of the IC. The number of diodes on the IC may beapproximately equal to n*(n−1), where n is the number of IC terminals.

[0029] In operation 320, the diodes are calibrated to determine theirelectrical characteristics across a range of possible operatingtemperatures. The calibration helps to determine voltage or currentvalues that may be safely applied to a thermal diode. Calibration may beaccomplished by soaking the die in a chemical bath at differenttemperatures. Moreover, the effective series resistance of the diode isalso calibrated. As stated above, the effective series resistance ofdiodes may be caused by long traces. For one embodiment of theinvention, the traces are copper wires. For another embodiment of theinvention, the traces are aluminum wires.

[0030] The resistance of copper and aluminum wires vary withtemperature. Thus, the relationship of the trace resistivity and itscorresponding temperature may be defined by a curve. This curve may havea known series resistance value R_(s0) at a temperature T₀, a knownseries resistance value ρ₀ at 0 Kelvin, and a slope M. Series resistanceR_(s) may be represented by: $\begin{matrix}{{R_{S} = {R_{S0}\left( {1 + \frac{T - T_{0}}{T_{0} + \frac{\rho_{0}}{m}}} \right)}},{*{All}\quad {temperatures}\quad {are}\quad {in}\quad {{Kelvin}.}}} & \left( {{equation}\quad 6} \right)\end{matrix}$

[0031] Following operation 320, a known current i₁ is sent through adiode on the IC in operation 330. The current is sent through the diodethrough an IC terminal and allowed to return via another IC terminal. Aspreviously discussed, it is possible to have two diodes having oppositepolarity coupled between any two IC terminals. Thus, the selected diodeis determined by which IC terminal the current is sourced since currentwill not conduct across a reverse biased diode.

[0032] The voltage drop v₁ is then measured across the diode's terminalsin operation 340. A second set of current and voltage values, i₂ and v₂,may then be obtained. To calculate the temperature of the diode inoperation 350, the voltage drop across the diode may be represented by:$\begin{matrix}{v = {{\left( \frac{nkT}{q} \right){\ln \left( \frac{i}{I_{s}} \right)}} + {i \cdot R_{S}}}} & \left( {{equation}\quad 7} \right)\end{matrix}$

[0033] which includes the effective series resistance. Accordingly, thetemperature equation of equation 4 may be rewritten as: $\begin{matrix}{T = {\left( \frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right){\left( {v_{1} - v_{2} - {R_{S}\left( {i_{1} - i_{2}} \right)}} \right).}}} & \left( {{equation}\quad 8} \right)\end{matrix}$

[0034] Equation 8 assumes that the series resistance of the trace isapproximately equal in temperature as the diode being measured. Equation8 may be rewritten as: $\begin{matrix}{T = {{\left( \frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right)\left( {v_{1} - v_{2}} \right)} - {\left( \frac{{qR}_{S}}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right){\left( {i_{1} - i_{2}} \right).}}}} & \left( {{equation}\quad 9} \right)\end{matrix}$

[0035] From equation 6, R_(s), may alternatively be represented by:$\begin{matrix}{R_{S} = {{R_{S0}\left( {1 + \frac{T_{0}}{T_{0} + \frac{\rho_{0}}{m}}} \right)} + {{T\left( \frac{R_{S0}}{T_{0} + \frac{\rho_{0}}{M}} \right)}.}}} & \left( {{equation}\quad 10} \right)\end{matrix}$

[0036] The equation 10 can be substituted into equation 9 such that$T = {{\left( \frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right)\left( {v_{1} - v_{2}} \right)} - {\left( \frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right)\left( {i_{1} - i_{2}} \right)\left( {{R_{S0}\left( {1 - \frac{T_{0}}{T_{0} + \frac{\rho_{0}}{M}}} \right)} + {T\left( \frac{R_{S0}}{T_{0} + \frac{\rho_{0}}{M}} \right)}} \right)}}$

[0037] Resolve for T: $\begin{matrix}\begin{matrix}\begin{matrix}{T = {{\left( \frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right)\left( {v_{1} - v_{2}} \right)} - {\left( \frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right)\left( {i_{1} - i_{2}} \right)\left( R_{S0} \right)\left( {1 - \frac{T_{0}}{T_{0} + \frac{\rho_{0}}{M}}} \right)} - {{T\left( \frac{R_{S0}}{T_{0} + \frac{\rho_{0}}{M}} \right)}\left( \frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right)\left( {i_{1} - i_{2}} \right)}}} \\{{T\left( {1 + {\left( \frac{R_{S0}}{T_{0} + \frac{\rho_{0}}{M}} \right)\left( \frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right)\left( {i_{1} - i_{2}} \right)}} \right)} = {{\left( \frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right)\left( {v_{1} - v_{2}} \right)} - {\left( \frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right)\left( {i_{1} - i_{2}} \right)\left( R_{S0} \right)\left( {1 - \frac{T_{0}}{T_{0} + \frac{\rho_{0}}{M}}} \right)}}}\end{matrix} \\{T = \frac{{{\left( \frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right)\left( {v_{1} - v_{2}} \right)} - {\left( \frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right)\left( {i_{1} - i_{2}} \right)\left( R_{S0} \right)\left( {1 - \frac{T_{0}}{T_{0} + \frac{\rho_{0}}{M}}} \right)}}}{1 + {\left( \frac{R_{S0}}{T_{0} + \frac{\rho_{0}}{M}} \right)\left( \frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}} \right)\left( {i_{1} - i_{2}} \right)}}}\end{matrix} & \left( {{equation}\quad 11} \right)\end{matrix}$

[0038] Alternatively, equation 11 can be expressed as: $\begin{matrix}\begin{matrix}{T = \frac{\left( {v_{1} - v_{2}} \right) - {\left( {i_{1} - i_{2}} \right)\left( R_{S0} \right)\left( {1 - \frac{T_{0}}{a}} \right)}}{\frac{1}{g} + {\left( \frac{R_{S0}}{\alpha} \right)\left( {i_{1} - i_{2}} \right)}}} \\{{{{where}\quad a} = {T_{0} + \frac{\rho_{0}}{M}}},{g = {\frac{q}{{kn}\quad {\ln \left( \frac{i_{1}}{i_{2}} \right)}}.}}}\end{matrix} & \left( {{equation}\quad 12} \right)\end{matrix}$

[0039] Equation 12 compensates for effective series resistance andeffective series resistance changes due to thermal variation. Note thatthe values v₁ and v₂ were obtained based on i₁ and i₂ inputs. The valuesfor R₀, T₀, ρ₀, and M were obtained during the series resistancecalibration of operation 320. By substituting these values into equation12, the temperature T of the diode may be calculated.

[0040] After the temperature calculation, operation 360 determineswhether there are further temperature measurement requests. If there arefurther measurement requests, a first known current and a second knowncurrent are sent through another diode in operation 330, the voltage ismeasured across the diode's terminals in operation 340, and thetemperature is calculated in operation 350. Otherwise, the process isterminated in operation 370.

[0041] For another embodiment of the invention, only one known currentis sent through a diode. The voltage is then measured and translatedinto a temperature by applying a linear approximation. The equation ofthe linear approximation may be determined by characterizing many partsto determine the average voltage response to temperature at a givencurrent.

[0042] For yet another embodiment of the invention, instead of sending aknown current through a diode and measuring the voltage across thediode's terminals as in operations 330 and 340 of FIG. 3, a knownvoltage source is placed across a diode's terminals in operation 330 andthe corresponding current is measured in operation 340. The remainingoperations of FIG. 3 remain the same.

[0043] In the foregoing specification the invention has been describedwith reference to specific exemplary embodiments thereof. It will,however, be evident that various modification and changes may be madethereto without departure from the broader spirit and scope of theinvention as set forth in the appended claims. The specification anddrawings are, accordingly, to be regarded in an illustrative rather thanrestrictive sense.

What is claimed is:
 1. An integrated circuit (IC) comprising: a firstdiode having an anode terminal and a cathode terminal; a first ICterminal coupled to the anode terminal of the first diode; a seconddiode having an anode terminal and a cathode terminal, wherein thecathode terminal of the second diode is coupled to the first ICterminal.
 2. The circuit of claim 1, further comprising: a second ICterminal coupled to the cathode terminal of the first diode and theanode terminal of the second diode.
 3. The circuit of claim 2, furthercomprising: a third diode having an anode terminal and a cathodeterminal, wherein the anode terminal of the third diode is coupled tothe second IC terminal; and a fourth diode having an anode terminal anda cathode terminal, wherein the cathode terminal of the fourth diode iscoupled to the second IC terminal.
 4. The circuit of claim 3, furthercomprising: a third IC terminal coupled to the cathode terminal of thethird diode and the anode terminal of the fourth diode.
 5. The circuitof claim 4, further comprising: a fifth diode having an anode terminaland a cathode terminal, wherein the anode terminal is coupled to thefirst IC terminal and the cathode terminal is coupled to the third ICterminal.
 6. The circuit of claim 5, further comprising: a sixth diodehaving an anode terminal and a cathode terminal, wherein the anodeterminal is coupled to the third IC terminal and the cathode terminal iscoupled to the first IC terminal.
 7. An integrated circuit (IC)comprising: a first diode, wherein the first diode is used to calculatea first temperature of the IC; a first IC terminal coupled to the firstdiode; a second diode coupled to the first IC terminal, wherein thesecond diode is used to calculate a second temperature of the IC; and asecond IC terminal coupled to the first and second diodes.
 8. Thecircuit of claim 7, wherein the first and second IC terminals are diebumps.
 9. The circuit of claim 8, further comprising: a first packagingpin coupled to the first IC terminal; and a second packaging pin coupledto the second IC terminal.
 10. A system comprising: an integratedcircuit (IC); n number of pins coupled to the IC, wherein n is aninteger greater than or equal to two; d number of diodes coupled to npins, wherein d is an integer equal to n*(n−1); and a power sourcecoupled to the IC, wherein the power source supplies a voltage to theIC.
 11. The system of claim 10, further comprising: a temperaturesensing circuit coupled to the IC to calculate a temperature of the IC.12. The system of claim 10, wherein the IC is a microprocessor.
 13. Thesystem of claim 11, wherein the temperature sensor reduces the voltagesupplied to the IC if the calculated IC temperature is greater than apredetermined value.
 14. A method comprising: placing a plurality ofdiodes on an integrated circuit (IC), wherein the IC has n terminals,wherein the plurality of diodes is approximately equal to n*(n−1); andcalibrating electrical characteristics of a first diode of the pluralityof diodes.
 15. The method of claim 14, further comprising: placing aknown voltage across a first terminal and a second terminal of the firstdiode; and measuring a current across the first diode.
 16. The method ofclaim 16, further comprising: calculating a temperature of the firstdiode.
 17. A method comprising: calibrating electrical characteristicsof a diode; sending a known first current through the diode andmeasuring a first voltage across the diode; sending a known secondcurrent through the diode and measuring a second voltage across thediode; and calculating the temperature of the first diode, wherein thetemperature calculation comprises compensation for an effective seriesresistance.
 18. The method of claim 17, further comprising: sending aknown current through a second diode of the plurality of diodes to makefurther temperature measurements.